In a conventional pulsed radar level gauge, constant power pulses are transmitted into the tank with a repetition frequency in the range 100 kHz to a few MHz. The pulses can be DC pulses or pulses modulated on a microwave frequency carrier wave. The pulse can be guided by a wave guiding structure into the tank, or be allowed to propagate freely. On the receiver side, a reflection from the interior of the tank is received, and a low frequency analogue tank signal is formed and then digitized to form a digital time domain reflectometry (TDR) signal.
Before the A/D-conversion, the analogue tank signal is amplified, partly with a ramp gain function and partly with a constant gain. The reason for this amplification is to match the received signal to the dynamics of the A/D-converter.
First of all, it is important to avoid saturation of the A/D-converter. However, it is also advantageous if as much as possible of the dynamic range of the A/D-converter is effectively utilized, as this increases the resolution of the digital signal, enabling a more exact identification and location of echoes. Therefore, the amplification of the tank signal is ideally arranged to ensure that all the peaks in the amplified tank signal are as close as possible to the maximum level of the A/D-converter, without exceeding it.
The ramp gain function suppresses the early portion of the tank signal (corresponding to reflections close to the top of the tank) while amplifying the final portion of the tank signal (corresponding to reflections close to the bottom of the tank). The reason for the gain function is that reflections occurring close to the top of the tank are stronger than reflections occurring close to the bottom of the tank. In a guided wave RLG, this is because pulses reflected close to the bottom of the tank travel a longer distance in the wave guide and thus are dampened to a larger extent. In a free propagating RLG, this is mainly because the emitted pulse is spread more before being reflected. The ramp thus serves to compensate for this effect of varying signal strength, so as to “even out” any peaks occurring in the tank signal, enabling a better utilization of the dynamic range of the A/D-converter. Such a linear gain may be calibrated during manufacturing, but will then remain unaltered.
The constant gain is used to ensure that the strongest echo in the tank signal is as close to the maximum level of the A/D-converter as possible. This constant gain can be controlled by software in the gauge.
A drawback with the above described solution is that the received signal and any noise are amplified an equal amount. Therefore, the signal-to-noise ratio of the tank signal is not improved, and leads to an imperfect utilization of the dynamic range of the A/D-converter.
An additional drawback is that the linear gain function on the receiver side will distort the tank signal, as the rising edge and falling edge of a peak will be amplified with different gains. Such distortions can lead to erroneous determination of the peak location, and must be compensated for later on in the processing, leading to more complex processing. Yet another drawback is that the gain function cannot compensate for variations in the actual conditions in the tank.